This invention relates to an optimal acoustic wave propagation orientation on an ST Cut Quartz Crystal for surface acoustic wave (SAW) device application.
In this application an "ST Cut Quartz Crystal" is defined as a quartz crystal having standard Euler angles lambda (.lambda.) and mu (.mu.) and does not include the wave propagation angle theta (.theta.), on the cut.
A wide variety of SAW devices are now produced for use in the VHF and UHF range and include filters, resonators, delay lines, convolvers, impedance elements and various types of sensors.
SAW devices typically involve interdigitated electrodes formed on some type of piezo-electric substrate such as quartz, lithium niobate and the like.
The orientations for surface acoustic wave propagation for any given crystal substrate are completely defined by the Euler angles, lambda, mu and theta. The first two angles, lambda and mu, define the crystal cut and the third angle, theta, defines the direction of acoustic wave propagation in that cut. Thus, acoustic wave propagation orientation on the crystal is defined by a unique set of all three Euler angles. (See Goldstein, H., Classical Mechanics, New York, (1950) (Addison-Wesley).
For a variety of surface wave materials and particular crystal cuts of interest, various surface acoustic wave propagation properties can be tabulated. Such tabulation includes velocity, coupling coefficient, electromechanical power flow angle curves, and magnitude and phase of the free surface mechanical displacements and electric surface potential. (See Microwave Acoustic Handbook IA and 2, Slobodnik et al, Air Force Cambridge Research Labs, October 1973 and October 1974.) Such data as set forth in these handbooks can be used a guide to selecting appropriate directions for wave propagation and orientation in the crystal necessary to achieve the desired transduction characteristic. Further, principal properties of interest include the effective centers of transduction, centers of reflection, temperature coefficient of delay, spurious bulk responses, diffraction characteristics, effective dielectric constants and the like.
The present invention relates to a newly discovered Quartz Crystal cut having a substantially planar surface defined by the Euler angles lambda (.lambda.) equal about 0.degree., mu (.mu.) equal about 132.75.degree. and theta (.theta.) equal about plus or minus 25.degree.. This crystal has the advantage of having Euler angles lambda (.lambda.) and mu (.mu.) identical to those of the standard ST cut crystal. Thus the normal ST cut wafer can be used with a newly discovered wave propagation angle approximately plus or minus 25.degree. from the x-axis. It has excellent coupling between the voltage on the electrodes and the acoustic waves in the crystal itself, has low beam steering and good temperature stability. It can be used for forming a SAW device used as an impedance element, a coupled resonator, or a filter.
Further, this cut and novel wave propagation direction allows the construction of a two electrode per wavelength transducer with unidirectional characteristics. It also allows the construction of a two electrode per wavelength transducer with a desired symmetrical input conductance function and a flat susceptance region. See U.S. Pat. No. 4,599,587 issued July 8, 1986 and entitled Impedance Element for a further description of another embodiment of such a transducer.